'IH'I The American Geologist. April, 1894 
the more recent writers, by Posepny, who has introduced the 
terms idiogenous and xenogenous to designate those two 
classes. It seems to the present writer, however, that in a 
broad view of the field this distinction disappears or becomes 
of subordinate importance. 
Strictly speaking, nearly all, if not quite all. known rocks 
are secondary. This is generally conceded for the sediment- 
ary rocks; but, following Iddings and other recent writers, it 
must be admitted also for the igneous rocks. A dike is a more 
or less highly differentiated portion of some deep-seated 
magma, and it is clearly newer than the walls. An ascension- 
ist. certainly, can say nothing more for a vein save that while 
in formation of the dike heat is the chief agent, with water 
cooperating (aqueo-igneous fusion), in the formation of the 
vein water is the chief agent, with heat cooperating (igneo- 
aqueous solution). The greater mobility of the solution per- 
mits a higher degree of concentration, but concentration is a 
universal process and participates in the formation of erup- 
tive masses and sedimentary deposits, as well as of veins. Jn 
other words, solidification and deposition, or the rock-making 
processes, invoke in every instance differentiation and con- 
centration: while liquefaction, or the destruction of rocks, 
whether aqueous or igneous, implies just as universally the 
undoing of differentiation or dissipation of matter. It appears 
illogical, therefore, to regard one class of rocks (vein rocks) 
as fundamentally distinguished by the fact that their forma- 
tion involves the concentration of their component minerals. 
The concentration is quite as marked in the formation of a 
bed of sand. clay, limestone, gypsum, chert or iron ore; and, 
if less marked, it is certainly not less true in the formation of 
a dike of granite or trap. In the opinion of the writer, no 
sharp line of demarcation can be drawn between dikes and 
veins, and veins are clearly entitled to some degree of recog- 
nition in the lithological classification. In a broad view of 
the early history of the earth all the sedimentary rocks are. 
of course, secondary with reference to the primitive igneous 
crust, but so are the igneous rocks with which we are now 
acquainted. ■ Probably none of the igneous which have been 
studied are truly primitive; and their derivation in some 
cases from sediments is claimed by manv able observers. 
